This invention relates to an apparatus and method for quantitatively analyzing a sample containing vaporizable molecules. The invention further relates to an apparatus and method for quantitatively analyzing organic pollutants such as polycyclic aromatic hydrocarbons.
One of the most difficult problems in analytical chemistry in recent years has been the quantitative analysis of minute amounts of organic pollutants. These pollutants derive from a variety of sources including energy resources, the chemical industry, and agricultural land. One significant class of organic pollutants is the polycyclic aromatic hydrocarbons (PAH's) generated from coal, synthetic fuel and shale oil. These compounds may have mutagenic and carcinogenic properties which are strongly dependent on isomeric structure and substitution; thus it would be highly desirable to be able to quantitatively analyze a sample for substitutional isomers of PAH's. However, substitutional isomers of PAH's often have very similar chemical and physical properties which greatly complicates their separation, identification, and quantitation.
Limited success on the quantitative analysis of organic pollutants has been achieved through gas chromatography (GC). Gas chromatography is a well-known and versatile tool for the separation and quantitative analysis of vaporizable compounds. In a typical gas chromatograph, a vaporizable sample is injected into a long narrow column containing a stationary liquid phase supported on an inert solid matrix. The sample is vaporized at the injection port and carried through the column by an inert mobile gaseous phase. The components of the sample are fractionated as a consequence of partition between the liquid and gaseous phases as they migrate along the column. The rate at which the various components migrate along the column depends upon their tendency to dissolve in the stationary liquid phase. As the components are eluted from the column they are detected by any of a variety of standard methods, such as thermal conductivity or flame ionization. The detector sends a signal to a chart recorder which records each eluted compound as a peak. Qualitative identification of the components is based upon the retention times, defined as the time required for peaks to appear at the end of the column. Quantitative data are obtained from evaluation of peak areas.
It may be seen that the success of a chromatographic separation depends on the different solubilities of the components of the mixture to be analyzed in the stationary liquid phase. Because substitutional isomers of PAH's often have almost identical solubilities, they usually cannot be resolved on a standard gas chromatograph. Thus gas chromatography is not satisfactory as an analytical tool for many PAH mixtures.
It is well known that most PAH's as well as other .pi.-electron molecules have fluorescent spectra characteristic of their structure and substitution. However, conventional room temperature solution spectra of many PAH's are not sufficiently well resolved to permit identification of the constituents of a complex mixture. This is due to the broad fluorescence vibronic bands produced, in part, by thermal molecular motions and coupling with liquid phonon modes. Even in the gas phase, PAH spectra are broad and complicated; in this case due to spectral congestion of rotational, sequence and hot bands. Thus ordinary fluorescence spectra can not be used to analyze gaseous PAH eluants from a gas chromatograph.
Rotationally cooled laser induced fluorescence (RC-LIF) is a technique used to study interacting molecular systems cooled to near 0.degree. K. in a supersonic molecular beam. By this method, either a gas sample or a liquid or solid sample with a high vapor pressure is placed in a chamber to provide a reservoir of gaseous sample molecules. The chamber may be heated if necessary to increase the gas pressure of the sample molecules. The sample molecules are seeded into a flow of helium, and the seeded helium is allowed to freely expand through a very small orifice in the reservoir chamber into a vacuum chamber to form a supersonic molecular beam. Within the beam a finite number of binary collisions occur which narrow the velocity distribution of the jet and cool the vibrational and rotational degrees of freedom of the seed molecules. By this method, the supersonic beam provides an intense source of seed molecules traveling in a vacuum with an extremely narrow velocity distribution, completely isolated from other particles and cooled to such an extent that the excited rotational and vibrational levels of the seed molecules are substantially depopulated. High resolution dye lasers may then be used to induce electronic transitions in the seed molecules with near unit probability. In those cases in which the laser-excited molecule fluoresces, a greatly simplified high-resolution spectrum can be obtained, with rotational and vibrational absorptions almost completely eliminated. The high sensitivity of fluorescent detection together with the high efficiency of laser excitation combine to produce large signals in spite of the otherwise prohibitively low density of the molecular beam.
RC-LIF has been used to obtain new spectroscopic data on a number of molecules. The theory and applications of RC-LIF are discussed in detail in "Laser Spectroscopy in Supersonic Jets" by Levy et al, Chemical and Biochemical Application of Lasers, Vol. II, edited by C. Bradley Moore, 1977, Academic Press, pages 1-41. Despite the tremendous potential of this technique, it has heretofore been limited to the acquisition of physical and spectroscopic data. Because the seeded beam must be continuously emitted from a reservoir, it has been impossible to quantify the amount of fluorescent seed material in the beam. In addition, it may require several hours to attain a spectrum of a single sample. Therefore this method has heretofore been unavailableas a tool for quantitative chemical analysis.